1. Field of the Invention
The present invention relates to a level shifter and a projector.
2. Description of the Related Art
An example of conventional means for transmitting data such as images from an external device to a projector is an interface (VESA standard) standardized by the Video Electronics Standards Association (VESAC)). The use of such means according to the standard enables the projector to receive data such as images from various external devices. Some interfaces standardized by VESA perform bidirectional Extended Display Identification Data (EDID) communication, to establish inter-device connection. The EDID communication is standardized by VESA as the Display Data Channel (DDC). DDC is based on the Inter-Integrated Circuit (I2C) which is a standard serial bus protocol for bidirectional communication employed in various electronic devices. DDC includes Serial Data Line (SDA), Serial Clock Line (SCL), and a +5 V power signal defined in I2C. When a device is connected and power is turned on, +5 V power is supplied and then EDID communication is performed by SDA and SCL according to the DDC specifications.
However, there are also external devices whose interfaces do not comply with the VESA standard in terms of voltage level or power voltage stability. To support such external devices, the projector needs to have a level shifter. For example, Japanese Patent Application Laid-Open No. 2000-307413 discloses a level shifter applicable to open-drain interface lines.
This is effective if the power voltage supplied to the interface circuit of the external device has sufficient stability. If not, however, a problem in that the amplitude of the signal waveform or the maximum value of the signal waveform varies with time arises.